Process for the dissolution of copper metal

ABSTRACT

Process for producing a copper-containing aqueous solution, in which a copper mass is dissolved in the presence of an oxidant in an aqueous leach liquor containing monoethanolamine and (HMEA) 2 CO 3 . The leach liquor is produced by partially carbonating the monoethanolamine.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a process for the dissolution ofcopper metal. More particularly, the invention relates to a process forproducing a copper-containing aqueous solution by dissolving copper inthe presence of an oxidant in an aqueous leach liquor containingmonoethanolamine and (monoethanolammonium)₂ carbonate (HMEA)₂CO₃.

[0003] 2. Description of the Prior Art

[0004] It is known to employ copper-containing aqueous solutions asbiocidal fluids, for example, for the pressure treatment of lumber andfor water purification. Examples of such fluids and uses thereof may befound, for example, in U.S. Pat. Nos. 4,929,454 and 6,294,071. Thecopper-containing solutions may be formulated, for example, bydissolving copper in aqueous solutions containing alkyl amines or alkylhydroxy amines, such as 2-hydroxyethylamine.

[0005] Alternatively, copper-containing solutions may be produced byreacting copper oxide with chromic acid and arsenous acid to produce asolution of the copper with chrome and arsenic. The solution issubsequently diluted with water and the resulting aqueous solution maybe injected into wood under pressure.

[0006] This chromated copper arsenate (“CCA”) is the primary additiveused in the treatment of wood against termite and other biologicalinfestation. Although the CCA is very effective, it has come underincreased pressure because of the environmental concerns associated withchromium and arsenic.

[0007] A new generation of pesticide is now emerging that appears to beefficacious, and which relies on the use of copper (in larger quantitiesthan in the CCA) in combination with other pesticidal components, suchas quaternary amines and triazoles. The copper is typically applied as asolution of the monoethanolamine complex of copper carbonate or borate.The commercial form of the copper concentrate usually contains about 100to 130 g/l copper which is diluted with water prior to injection intothe wood.

[0008] The copper complex is typically produced commercially by thedissolution of basic copper carbonate in a solution of monoethanolamine(MEA), followed by further carbonation or addition of boric acid. Thereactions can be approximately represented by the following equations:

CuCO₃.Cu(OH)₂+7MEA→Cu(MEA)_(3.5)CO₃+Cu(MEA)_(3.5)(OH)₂

Cu(MEA)_(3.5)(OH)₂+CO₂(or Boric acid)→Cu(MEA)_(3.5)CO₃+H₂O

[0009] The production of the copper carbonate precursor has its ownproduction and raw material costs, and a brine waste is generated whichgives rise to environmental concerns. A more efficient process might beto produce the complex without the aid of an isolated precursor. It isknown from the prior art that ammonia and carbon dioxide in water can beused to dissolve copper metal with oxygen from air as the oxidant. Thisis represented by the following equation:

Cu+2NH₃+(NH₄)₂CO₃+1/2O₂→Cu(NH₃)₄CO₃+H₂O

[0010] The reaction proceeds well and has been the basis for copperdissolution in several commercial facilities. However, if the ammonia isnot initially carbonated, the kinetics are very poor which makes theprocess unattractive from a commercial standpoint.

[0011] A need exists for a more efficient process for producingcopper-containing aqueous solutions, suitable for use in thewood-treatment industry. The present invention seeks to fill that need.

SUMMARY OF THE INVENTION

[0012] It has been discovered, according to the present invention, thatit is possible to efficiently produce copper-containing solutionsdirectly without initially producing or isolating a precursor such ascopper carbonate. The present invention accordingly provides a processfor producing a copper-containing aqueous solution, wherein a coppermass is dissolved in the presence of an oxidant in an aqueous leachliquor containing monoethanolamine and (monoethanolammonium)₂ carbonate(HMEA)₂CO₃, the leach liquor being produced by partially carbonating themonoethanolamine. According to the process, copper-containing solutionmay be produced in 8-12 hours or less, more usually within about 8 hoursor less.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The present invention will now be described in more detail withreference to the accompanying drawings, in which:

[0014]FIG. 1 is a schematic depiction of a batch dissolver to be usedfor performing the present invention in a batch-wise manner;

[0015]FIG. 2 is a plot showing the copper dissolution in the batchprocess;

[0016]FIG. 3 is a plot showing the copper dissolution rate as a functionof time;

[0017]FIG. 4 is a schematic for continuous production of MEA complex ofcopper solution;

[0018]FIG. 5 is a plot showing the copper dissolution rate versus airflow rate in a continuous process at pH 9.8;

[0019]FIG. 6 is a plot showing the copper dissolution rate versus airflow rate in a continuous process at pH 9.5;

[0020]FIG. 7 is a plot showing the copper dissolution rate as a functionof pH.

DETAILED DESCRIPTION OF THE INVENTION

[0021] Referring to FIG. 1, there is shown a batch dissolver, generallyreferenced 2, having a chamber 4, with a false bottom 6, and an airsparger 8 located below the false bottom. The chamber is surrounded by aheating coil 10 and has a top 12 through which extend a thermocouple 14and an air inlet 16. The thermocouple is connected to a temperaturecontrol unit 18 and supplies heat to the heating coil 10 through conduit20. Leach liquor 22 is circulated through the chamber by circulationpump 24 through lines 26, 28. Copper metal 30, typically scrap copper,is present in the chamber and immersed in the leach liquor to enable thedissolution to occur.

[0022] The term “copper” as used herein means copper metal, includingscrap copper, such as for example copper wire, copper clippings, coppertubing, copper cabling and/or copper plate, compounds of copper, such ascopper oxide, and/or mixtures of copper metal and copper compounds.

[0023] The term “copper mass” as used herein refers to copper metal in aform which, when present in the chamber, is permeable to the leachliquor and which presents high surface area for contact with the leachliquor to thereby expedite dissolution of the copper. The copper massmay be present for example as a three-dimensional open permeablenetwork, such as a bale of scrap copper comprised of copper wire, coppertubing, copper cabling. copper plates, providing voids between thecopper pieces to allow free flow and maximum contact of the leach liquorwith the copper. A bale may have a volume of for example about 25-100cubic fee. Alternatively, the copper mass may be present in the chamberas smaller irregular shaped pieces resembling “popcorn” (“blister shot”)having an average dimension of about 1-3″, which allow for goodpermeation of the leach liquor between and around the copper pieces toexpedite dissolution thereof. Typically, the ratio of copper surfacearea to volume of leach liquor for this process versus a standardagitated reactor is about 10-20:1, for example about 15:1.

[0024] According to the process, the copper mass is dissolved in thepresence of an oxidant in an aqueous leach liquor containingmonoethanolamine and (HMEA)₂CO₃. Typically, for a 1 liter chamber, theair flow SCFH ranges from about 2-20, for example 3-10. The air flowSCFH will increase as the volume of the chamber increases.

[0025] The leach liquor is produced by partially carbonating themonoethanolamine and may be generated externally of the dissolver or insitu in the chamber through addition of carbon dioxide to themonoethanolamine/water solution by sparging or bubbling into thechamber. Usually, the leach liquor is produced externally of the chamberand introduced into the chamber into contact with the copper asrequired, or re-circulated as necessary.

[0026] The equation that represents the overall reaction is as follows:

Cu+1.5MEA+(HMEA)₂CO₃+1/2O₂→Cu(MEA)_(3.5)CO₃+H₂O

[0027] The equation that represents the in situ or external partialcarbonation of the monoethanolamine is as follows:

3.5MEA+CO₂+H₂O→1.5MEA+(HMEA)₂CO₃.

[0028] The amount of carbon dioxide introduced during the process iscontrolled such that partial carbonation occurs to form (HMEA)₂CO₃.Typically, the carbon dioxide is present in an amount of about 5-30% byweight, for example about 8-12% by weight.

[0029] The MEA is usually present in an amount of about 30-40 wt %, moreusually 35-38 wt %. A typical aqueous leach solution of comprises about36 wt % MEA and about 10% by weight carbon dioxide.

[0030] The monoethanolamine complex of copper carbonate solutions aretypically prepared by dissolving the copper mass in amonothanolamine/CO₂/H₂O solution. The dissolution may be carried out ina batch dissolver (see FIG. 1), or may be performed as a continuousprocess in towers packed with copper (see FIG. 4). Typically, the copperand MEA/CO₂/H₂O solution are charged into the dissolver, and thecirculation pump, air-flow and temperature controller are actuated.Examples of conditions are given in Table 3 below.

[0031] The present inventors have discovered that it is not necessary toutilize precursors, such as copper carbonate which is expensive. Thedissolution of the copper metal may be achieved in the presence of MEA,(HMEA)₂CO₃ and an oxidant at elevated temperature, without the need forthe addition of ammonium compounds such as ammonium hydroxide,fungicidal anions, polyamines, carboxylic acids, alkali metal hydroxidessuch as sodium hydroxide, and/or alcohol-based solvents.

[0032] The dissolution of the copper metal is performed in the presenceof an oxidant. Typically the oxidant is an air and/or oxygen, mostusually air sparging.

[0033] The leach solution is typically re-circulated in the reactor.Most typically, the re-circulation is carried out at a constant rate,and may be, for example, a constant rate of about one-tenth of the leachsolution volume per minute.

[0034] The process may be carried out at atmospheric pressure and at atemperature of 25-100° C., for example 45-65° C. Typically, thetemperature is maintained at 45-55° C.

[0035] The pH is typically maintained in the basic region, i.e. greaterthan 7, and is usually from about 8.0-11.3, more usually 9-10. The pH isusually maintained by addition of carbon dioxide as acid, or MEA asbase.

[0036] The reaction proceeds slowly where there is a small surface areaof copper available to contact the leach solution. For example, if thecontact area of the copper metal to leach solution is doubled, the rateof the process doubles assuming adequate air-distribution.

[0037] It has been found that an aerated packed tower containing coppermetal and circulating leach liquor is the most preferred method forcommercial purposes. The advantage of using a packed tower is that itmaintains a relatively high surface area to solution volume of coppermetal.

[0038] According to another embodiment of the present invention, it ispossible to utilize a reactor that contains a bed of copper and can berotated while being heated and aerated.

[0039] Dissolving studies were conducted either batch-wise orcontinuously. FIG. 1 shows a conventional batch dissolver used for thebatch-wise operation. FIG. 4 shows a continuous dissolution process. Theconditions used in the batch experiments are given in Table 1. Thetemperature was maintained at 45-55° C. TABLE 1 EXP. Leach Solution # IDAmine, % CO₂, % Volume, ml Cu Charge, g 1 MEA-CO₂ 36.7 12 600 400solution 2 MEA-CO₂ 35.9 13 200 200 solution

[0040] The leach solutions are typically re-circulated in the reactor.Most typically, the re-circulation is carried out at a constant rate ofabout one-tenth The solution concentration of copper (g/l) as a functionof dissolution time is shown in Table 2: TABLE 2 EXP. Dissolution Time,hour # 1 2 3 4 5 6 7 8 1 3.7 15.7 41.3 67.8  88.2 100.3 2 5.4 16.8 55.785.1 102.5 119.2 126.4 136.1

[0041] In experiments 1 and 2 reported above, average copper dissolutionrates of about 17 g/l-hr were achieved over the course of theexperiments. At those rates, the process is viable commercially. Rawmaterial costs, processing costs and waste are significantly reducedover the conventional process using copper carbonate.

[0042]FIG. 4 is a schematic for the continuous production of MEA complexin solution. The dissolver, generally referenced 32, has a chamber 34,with a false bottom 36, and an air sparger 38 located below the falsebottom. The chamber is surrounded by a heating coil 40 and has a top 42through which extend a thermocouple 44 and an outlet 46. Thethermocouple is connected to a temperature control unit 48 and suppliesheat to the heating coil 40 through conduit 50. Leach liquor 52 iscirculated through the chamber by circulation pumps 54 through lines 56,58. A copper mass 60 is present in the chamber and immersed in the leachliquor to enable the dissolution to occur. The system is also providedwith a pH control 62 connected to a gravity controller 64 into whichcarbon dioxide is admitted from tank 66. Carbon dioxide off-gas isdirected through line 46 to a carbon dioxide scrubber 68. Carbon dioxidefrom the scrubber 68 is the fed to chamber 70 containing MEA and waterwhich is pumped via pump 72 to chamber 34. The system also comprises anoxidation chamber 74 into which oxygen is admitted via line 76. Productenters at line 78 and following oxidation exits via line 80 and istransferred to product storage.

EXAMPLES

[0043] Examples of the process according to the present invention willnow be described.

Example 1

[0044] Batch Preparation of Monoethanolamine Complex of Copper Carbonate

[0045] Monoethanolamine complex of copper carbonate solutions wereprepared by dissolving a copper metal mass in monothanolamine/CO₂/H₂Osolution in the batch dissolver in the presence of air sparging and atan elevated temperature. FIG. 1 shows a conventional batch dissolverused for the batch-wise operation.

[0046] Three experiments were conducted using the batch dissolver shownin FIG. 1. In each experiment, about 1200 g copper and 1 literMEA-CO₂—H₂O solution were charged into the dissolver. The circulationpump, airflow and temperature controller were then started. Theexperimental conditions are given in Table 3. TABLE 3 MEA-H₂O-CO₂Solution Air Flow Circulation Rate Exp ID MEA/H₂O¹ % CO₂ Sp.G SCFHTemperature ° C. ml/min 1 0.900/1.00 13.7 1.165 6.0 51 ± 1 182 20.733/1.00 14.1 1.160 6.0 51 ± 1 182 3 0.900/1.00 13.7 1.165 6.0 76 ± 1182

[0047] When temperature reached the target temperature, the first sampleof each batch was taken for analysis, and the timer was started.Complete results of these three dissolving batches are shown below, andare presented in FIGS. 2 and 3. FIG. 2 is a plot showing the copperdissolution in the batch process, and FIG. 3 is a plot showing thecopper dissolution rate as a function of time. TABLE 4 BatchDissolving-Experiment #1 Time, hour % Cu pH Sp.G. Copper, g/L 0 0.16 8.81.165  1.9 1  2.155 9.6 1.175  25.3 2 4.85 N/a 1.205  58.4 3 6.73  9.451.228  82.6 4 7.66 9.6 1.239  94.9 5 8.36 9.6 1.251 104.5 6 9.23 9.61.262 116.5 7 9.79 N/a 1.271 124.4 8 10.32  9.65 1.277 131.8

[0048] TABLE 5 Batch Dissolving-Experiment #2 Time, hour % Cu pH Sp.G.Copper, g/L 0 1.17 9.00 1.170 13.7 1 3.5  9.25 1.180 41.3 2 6.07 9.301.204 73.1 3 7.37 9.37 1.223 90.1 4 5 9.29 9.55 1.250 116.1  6 9.76 9.651.258 122.7  7 10.23  9.65 1.265 129.4  8 10.63  9.65 1.267 134.7 

[0049] TABLE 6 Batch Dissolving-Experiment #3 Time, hour % Cu pH Sp.G.Copper, g/L 0 1.175  9.45 1.175  13.8 1 7.054  9.80 1.208  85.2 2 8.66110.0 1.237 107.1 3 10.11  10.2 1.251 126.4 4 10.99  10.3 1.267 139.2

Example 2

[0050] A continuous dissolver assembly (see FIG. 4) was used in theexperiments described below. The assembly includes a one-liter sizepacked-tower dissolver (used in the batch dissolving experimentsdescribed above); a gravity controller, a temperature controller, a pHmonitor, an air flow meter a, circulation pump and a pump forsimultaneous withdraw and replenish of solutions. The gravity controllerheld about 1 liter of the product solution. The solution in the assemblywas circulated between the gravity controller and the dissolver at aconstant rate of 325 ml/min. Occasionally, CO₂ gas was bubbled throughthe bottom of the gravity control chamber to adjust the pH of thesolution. In all experiments described below, the reaction temperatureand specific gravity were controlled at 50±2° C. and 1.271±0.001 g/ml,respectively. During a continuous dissolving experiment, copper iscontinuously dissolved and results in a graduate increase of thespecific gravity of the copper-containing solution. When the gravityreaches a pre-set value, i.e. 1.272, it triggers a pump to withdraw theproduct solution and replenish MEA-CO2 solution simultaneously. Thecomposition of the MEA-CO2 solution used in all continuous dissolvingexperiments is the same as that of Exp#1 of batch process (Table 3). Thedissolver was charged on a daily basis with 1″ pieces of 11-13 AWG scrapcopper wires and maintained a total copper loading of 1100-1200 grams atany given period of the experiments. Results are shown below in Table 7along with the experimental conditions used, and are also presented inFIGS. 5-7. FIG. 5 is a plot showing the copper dissolution rate versusair flow rate in a continuous process at pH 9.8, FIG. 6 is a plotshowing the copper dissolution rate versus air flow rate in a continuousprocess at pH 9.5 and FIG. 7 is a plot showing the copper dissolutionrate as a function of pH. TABLE 7 Conditions and Results of ContinuousDissolving Experiments Copper Duration Air Flow MEA-Cu-CO₂ SolutionDissolution Hour pH SCFH Volume, ml Copper, % g/l/hr 2.13 9.8 6.0 125 10.61 7.91 2.13 9.8 3.0 85 10.41 5.28 0.65 9.8 8.0 41 10.41 8.35 3.359.5 6.0 183  10.15 7.05 0.64 9.5 10.0  46 10.15 9.27 0.78 9.5 4.5 3510.15 5.79 4.1  9.1 6.0 123   9.575 3.65

[0051] While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiment, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

What is claimed is:
 1. A process for producing a copper-containingaqueous solution, comprising dissolving a copper mass in the presence ofan oxidant in an aqueous leach liquor containing monoethanolamine and(HMEA)₂CO₃, wherein said leach liquor is produced by partiallycarbonating the monoethanolamine.
 2. A process according to claim 1,wherein said oxidant is selected from the group consisting of air andoxygen.
 3. A process according to claim 1, wherein the process iscarried out at a temperature of 40-80° C.
 4. A process according toclaim 3, wherein said temperature is 45-55° C.
 5. A process according toclaim 1, wherein said leach solution is re-circulated at a constantrate.
 6. A process according to claim 5, wherein said recirculation iscarried out at a constant rate of about one tenth of the leach solutionvolume per minute.
 7. A process according to claim 1, wherein thereaction is represented by the following equation:Cu+1.5MEA+(HMEA)₂CO₃+1/2O₂→Cu(MEA)_(3.5)CO₃+H₂O
 8. A process accordingto claim 1, wherein said partial carbonating of said themonoethanolamine is according to the following equation:3.5MEA+CO₂+H₂O→1.5MEA+(HMEA)₂CO₃.
 9. A process according to claim 1,wherein the carbon dioxide is present in an amount of about 5-30% byweight.
 10. A process according to claim 1, wherein the pH is 8.0-11-3.11. A process according to claim 1, performed as a batch process.
 12. Aprocess according to claim 11, wherein the average copper dissolutionrate is about 17 g/l -hour.
 13. A process according to claim 1,performed as a continuous process.